In general, a color cathode ray tube of the flat faceplate type, as shown in FIG. 1, includes a flat type shadow mask S made of an apertured metallic foil, which is welded to a support frame T under a predetermined tension and located alongside the inner surface of a flat faceplate P sealed to a funnel F. Flat faceplate CRT's are manufactured as, for example, 9-21 inch CRT screens. The thin foil type shadow mask S cannot easily maintain its shape, (i.e., its planarity) because of processing difficulties.
The prior art process for fixing the above-mentioned shadow mask is summarized as follows:
I) Heating the shadow mask for the thermal expansion in a heating apparatus.
II) Clamping the thermally expanded shadow mask to a fixture frame (not shown);
III) Cooling the clamped shadow mask to room temperature, thereby creating tension in the mask to be taut and planar;
IV) Welding the shadow mask to the support structure adjacent to the inner surface of the faceplate; and
V) Removing the fixture frame from the panel after trimming off the periphery of the shadow mask.
In the above-described steps, the shadow mask heating apparatus tends to thermally expand by heating the shadow mask. Clamping to the fixture frame by clip means provided on the fixture frame prevents the thermally expanded shadow mask from shrinking when subsequently cooled. In the taut state, the effective area of the shadow mask is welded to the support frame and finally trimmed to remove the fixture frame from the shadow mask.
In U.S. Pat. No. 4,772,238, there is disclosed a foil mask stretching apparatus 10, which is illustrated in FIGS. 3 and 4 of the present invention drawings. The stretching apparatus illustrated in FIG. 3 comprises upper and lower heated platens 11 and 12, each having a heating element 13 located therein and made of heat resistant steel. A shadow mask S is located between the platens 11 and 12 to be heated for thermal expansion.
Migrant particles are shed from the clamping machine, the clothing of the operator or other nearby pollution sources. These particles may become trapped between or on the platens 11 and 12, the shadow mask S. Such particles can plug the shadow mask apertures and also dent the mask when it is heated. In order to prevent the shadow mask from being damaged by migrant particles, recesses 12a are formed on the lower platen 12 so that the migrant particles fall in the recesses 12a.
However, when the shadow mask is heated to be thermally expanded, it can sag into the recesses 12a. When the mask sags excessively into recesses 12a, the mask cannot maintain the required planarity, even after cooling, and thus the mask remains in a deformed state.
Excessive sagging is somewhat prevented by matrix type mask support elements 12b provided on the top surface of the lower platens 12 and also made of heat resistant steel. Mask support elements 12b define a plurality of square type recesses 12a.
For heating, the shadow mask S is inserted between the platens 11 and 12, and then a voltage is applied to the heating elements 13 incorporated in the platens 11 and 12. Accordingly, the heat produced from the heating elements 13 is transmitted via the platens 11 and 12 to heat the shadow mask S.
This heat is transmitted to the mask S over recesses 12a by indirect conduction such as convection and radiation. However, the temperature on the portions of the mask located immediately above each recess 12a is substantially lower than the portion in direct contact with the mask support element 12b. Because of this conduction, enhanced by the flat surface of mask support elements 12b, the whole surface of the mask is not uniformly heated and undesired distortions and deformations still occur.